perf(sinan): remove unnecessary cwd's in FTP_SINAN

[luabida]

listing files in ftp server is very dependent on server connection. On tests with airflow, it showed inconstancy. Therefore, this PR tries to reduce the amount of server requests in the FTP_SINAN class

[luabida]

  _     ._   __/__   _ _  _  _ _/_   Recorded: 17:40:51  Samples:  1930
 /_//_/// /_\ / //_// / //_'/ //     Duration: 19.230    CPU time: 2.457
/   _/                      v4.4.0

Program: /home/luabida/Projetos/EGH/EpiGraphHub/containers/airflow/dags/brasil/sinan.py

19.227 <module>  sinan.py:1
├─ 18.069 task_flow_for  sinan.py:59
│  ├─ 17.605 FTP_SINAN.__init__  pysus/online_data/__init__.py:650
│  │  └─ 17.605 FTP.nlst  ftplib.py:547
│  │        [60 frames hidden]  ftplib, socket, .., re, sre_compile, ...
│  └─ 0.249 _TaskDecorator.__call__  airflow/decorators/base.py:310
│        [199 frames hidden]  airflow, abc, .., enum, logging, copy...
├─ 0.426 <module>  airflow/decorators/__init__.py:17
│     [1778 frames hidden]  airflow, flask, werkzeug, http, re, s...
├─ 0.411 <module>  epigraphhub/data/brasil/sinan/__init__.py:1
│     [3 frames hidden]  epigraphhub, ..
│        0.407 <module>  pysus/online_data/__init__.py:1
│        └─ 0.236 <module>  pandas/__init__.py:3
│              [584 frames hidden]  pandas, pyarrow, copy, .., textwrap, ...
└─ 0.233 <module>  airflow/__init__.py:18
      [871 frames hidden]  airflow, importlib, sqlalchemy, textw...

[github-actions[bot]]

:tada: This PR is included in version 0.9.0 :tada:

The release is available on:

• 0.9.0
• GitHub release

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